This invention is directed to a method and apparatus for foaming fluids and is more particularly directed to a method and apparatus for foaming fluids inside a pressurized chamber.
Hydraulic cement slurries are commonly utilized in subterranean well operations. For example, hydraulic cement slurries are used in primary cementing operations whereby strings of pipe, such as casings are cemented in wellbores. In performing primary cementing, a hydraulic cement slurry is pumped into the annular space between the walls of the wellbore and the exterior surfaces of pipe strings disposed therein. The cement slurry is permitted to sit in the annular space to form an annular sheath of hardened substantially impermeable cement therein. The cement sheath physically supports and positions the pipe in the wellbore and bonds the exterior surfaces of the pipe to the walls of the wellbore whereby the undesirable migration of fluids between zones or formations penetrated by the wellbore is prevented. Hydraulic cement slurries are also used in a number of other operations, including remedial cementing operations.
In carrying out primary cementing, as well as remedial cementing operations in wellbores, the cement slurries utilized must often be light weight to prevent excessive hydrostatic pressure from being exerted on subterranean formations penetrated by the wellbore. As a result, a variety of light weight cement slurries have been developed and used, including foamed cement slurries.
In addition to being light weight, a foamed cement slurry contains compressed gas which improves the ability of the slurry to maintain pressure and to prevent the flow of formation fluids into and through the slurry during its transition time, i.e., the time during which the cement slurry changes from a true fluid to a hard set mass. Foamed cement slurries often include various surfactants known as foaming agents to facilitate the foaming of the cement slurry when gas is mixed therewith. Other surfactants known as foam stabilizers for preventing the foam slurries from prematurely separating into slurry and gas components may also be added to the slurry. Foamed cement slurries are also advantageous because they have low fluid loss properties.
Because wellbores have different environmental conditions, laboratory tests are conducted on foamed cement slurries to determine certain characteristics of the slurries, such as compressive strength and fluid loss characteristics, to determine the fitness of the slurry for use in a particular well environment. The present manner of foaming and then testing a foamed fluid calls for placing the mixture, including the cement and any additives, such as foaming agents, in a stirring cell. The stirring cell is then sealed and pressurized with a gas, such as, but not limited to nitrogen. Paddles in the stirring cell are then activated and the mixture is agitated and stirred with the paddles until it is sufficiently foamed. The foamed fluid is transferred through high pressure lines to a separate pressure cell so that laboratory tests, such as, but not limited to, a fluid loss test, can be conducted. The foamed fluid may also be transferred to the pressure cell and cured at a high temperature while maintaining pressure, so that a compressive strength test may be conducted on the cured sample. Because it is necessary that pressure on the foamed fluid be maintained, there are several disadvantages to this method. First, the transfer process can cause the density of the foamed fluid to vary from sample to sample. In addition, the transfer process can be unsafe if inexperienced personnel are conducting the tests. Thus, there is a need for an improved apparatus and method for foaming and then testing foamed fluids containing cement, water, and other additives such as foaming agents, surfactants and/or stabilizers.
The present invention provides an improved method and apparatus for foaming and testing fluid slurries, more specifically foamed cement slurries, for use in well operations. The method comprises placing a mixture of at least water, cement and a foaming agent in a pressure cell, which may be referred to as a pressure vessel. The term water used herein includes both water and saltwater and other additives may be included in the mixture. The pressure vessel preferably is a cylindrical pressure vessel. The method further comprises injecting a gas into the pressure vessel and foaming the mixture to form a foamed cement slurry. The method further comprises conducting laboratory tests on the foamed slurry without transferring the slurry to a separate pressure vessel. The foaming step includes placing a perforated plate in the pressure vessel and moving the perforated plate, or disk, in the pressurized vessel through the mixture to activate the foaming agent and disperse gas bubbles therein, thereby foaming the mixture. Preferably, the vessel is rotated so that the perforated plate will move through the mixture between first and second ends of the vessel.
The laboratory test to be conducted may be a fluid loss test, a compressive strength test or other desired test. It is well known in the art that if the test to be conducted is a fluid loss test, the cylindrical pressure vessel will include a screen therein. Once the fluid is sufficiently foamed, a valve on the pressure vessel may be opened so that as the slurry contacts the screen, filtrates may pass therethrough and through the opened valve to allow fluid loss characteristics of the slurry to be measured. If the compressive strength test is to be conducted, the foamed fluid can simply be cured in the pressure vessel at a desired pressure and a desired elevated temperature for a period of time. The cured specimen can then be removed from the vessel and compressive strength tests may be conducted. If desired, a sleeve may be placed in the vessel to facilitate removal of the cured specimen. The fluid loss and compressive strength tests are to be conducted in accordance with API Recommended Practice 10B For Testing Well Cements (xe2x80x9cRecommended Practice 10Bxe2x80x9d).
The apparatus includes a pressure vessel of a type known in the art which is presently utilized for receiving a pressurized foamed fluid from a separate prior art vessel used to foam the fluid. Typical pressure vessels include cylindrical vessels valved to provide for the introduction of a high pressure gas, such as air or nitrogen, and also valved so that pressure can be relieved therefrom and fluid loss tests can be conducted or a foamed sample may be cured. The vessel will typically have a pressure gauge attached thereto so that the pressure inside the vessel can be monitored. The apparatus of the present invention includes a disk or plate with a plurality of openings therethrough. The disk is disposed inside the pressure vessel.
A motor having a shaft extending therefrom is attached to the pressure vessel. The motor can be of any type known in the art, and when activated will rotate the vessel at a desired revolutions per minute (rpm) of approximately 1 to 15 rpm, preferably at about 1.5 to 3 rpm, and more preferably at about 3 rpm. The disk in the pressure vessel will move through the fluid as the vessel is rotated. As the disk moves through the mixture, the gas in the vessel will react with the foaming agent to foam the mixture. Once the fluid is sufficiently foamed, laboratory tests, such as fluid loss and compressive strength tests can be conducted in accordance with Recommended Practice 10B.
It is therefore a general object of the present invention to provide an improved method and apparatus for foaming slurries and for testing the slurries. Other and further objects, features and advantages of the present invention will be readily apparent to those skilled in the art in view of the drawings herein, and a reading of the description of the preferred embodiments which follows.